Inserting frames:

You'll need to master the basics of the Animation Palette. First, change the frame-total to make room for the extra frames. Then, draw a selection box around all the keyframes on the Palette and manually drag them to the end of the timeline.

Doubling the frames:

Again, adjust the frame-total to make room for any changes (in this case, from 120 to 240). Then use the Retime Animation feature in the Animation menu, source frames being the original 1-120, destination frames being 1-240.